NL2018981B1 - Method and system for improving the surface fracture toughness of brittle materials, and a cutting tool produced by such method - Google Patents

Abstract

This invention concerns a method of improving the fracture toughness of a substrate made from a brittle material, such as a cermet. The method includes selecting a cermet which has a fracture toughness between 6 and 15 MPa.m1’2. The method includes the steps of applying a coating to the substrate material and creating a hardened surface layer by means of laser shock peening, thereby improving the fracture toughness and hardness of the substrate material. The invention also concerns a tool or tool insert made from a cermet and treated using the method according to the invention.

Description

Title: METHOD AND SYSTEM FOR IMPROVING THE SURFACE FRACTURE

TOUGHNESS OF BRITTLE MATERIALS, AND A CUTTING TOOL PRODUCED BY SUCH METHOD

Description

BACKGROUND TO THE INVENTION

This invention relates to a method and system for improving the surface fracture toughness of brittle materials, which, in turn, improves other mechanical properties such as abrasion resistance. In particular, but not exclusively, the invention relates to a method of inducing residual compressive stresses in brittle materials, preferably by means or laser shock peening. For example, the invention relates to a method of performing laser shock peening or cemented carbides, such as NbC and WC-based cermets, for improved cutting edge fracture toughness. The method also relates to a cemented carbide cutting element or cutting element insert which is manufactured using the method according to the invention.

Cutting, mining and other industrial abrasive tools are commonly made from hard, brittle materials such as cemented carbides, for example. Cemented carbides are well-known and originally consistent or micron sized tungsten carbide particles bonded with cobalt. An advantage of cemented tungsten carbide is that it is more wear resistant than steel while having a high toughness. As a result of these properties, cemented tungsten carbide has been used over the years in the manufacture of tools used in industrial applications where wear resistance and toughness are important criteria.

In an attempt to improve the mechanical, cutting properties of such tools, the sintering process, the composition of the material and / or the surface treatment or the material could be improved.

Known attempts to improve the composition of the materials have the use of cermets. Cermets are composite materials including ceramics, such as WC and / or NbC, embedded in a ductile metallic matrix, such as Co, Ni and / or Fe. Tungsten carbide (WC) - cobalt (Co) based cermets are commercially the most successful cermets due to the good combination of hardness, abrasion wear resistance, toughness fracture and strength. However, because of recent WC supply constraints and increasing cost, as well as poor chemical stability, particularly for machining or steel and cast irons, alternatives such as niobium carbide have been investigated. Niobium carbide (NbC) has similar hardness (about 19.6 GPa) to WC (about 22.4 GPa), a high melting point (3,522 ° C) which is good for high temperature applications, and low density (7.89 g / cm3). Niobium carbide has improved high temperature properties compared to WC, such as retention or hot hardness at elevated temperatures and good chemical stability, particularly when machining steels and cast irons. However, NbC-Co cermets produced by conventional liquid phase sintering (LPS) have lower harness and fracture toughness than WC-Co cermets, because of NbC grain growth. The poor fracture toughness (Kic) leads to mechanical failure, particularly spalling, at the NbC based tool inserts cutting edge as shown in Figure 1. The failure is typically due to the mechanical and thermal cycle loading during face milling. Figure 1 shows an optical image of an NbC-12Co (wt%) cutting tool insert in which the catastrophic failure or the cutting edge on (a) the flank and (b) the rake due to mechanical failure during face-milling or BS 1452 gray cast iron can be seen.

Although cemented carbides in use today are still predominantly tungsten carbide cemented with cobalt, many variations have been introduced. For example, titanium carbide and tantalum carbide by themselves or mixed with tungsten carbide have been used. In some instances chromium carbide is also added to the carbide mixtures. Other cementing alloys, such as alloys or nickel and iron, have also been proposed. It has also been suggested to apply coatings to cemented carbides in an attempt to improve their cutting properties.

Laser shock peening (LSP) is a well-known cold working process in which the surface of a material is treated with laser pulses to impart beneficial compressive residual stresses in the material so as to increase the resistance of the material to surface-related failures. The LSP process is one of the most advanced forms of peening, with several advantages about conventional mechanical peening process. The depth and magnitude of compressive residual stress generated with LSP is greater than mechanical peening. The LSP process uses high powered laser pulses which are forced onto the target to generate a rapid plasma expansion. An inertial confinement medium is used to confine and enhance the pressure of the plasma to achieve high pressure or several GPa. The high pressure acting over a short time interval drives a shock wave through the solid target with sufficient strength to exceed the materials dynamic yield strength, generating generating beneficial compressive residual stresses. These compressive stresses which inhibit crack propagation under static and cyclic loading, thus improving the fracture toughness (Kic) and fatigue life.

However, like mechanical shot peening, LSP is typically applied to metals since the generation of the residual stress is achieved by introduction or plasticity through the material's surface. Although there have been attempts to perform LSP on brittle materials, such as ceramics, they have been largely unsuccessful. It has been believed that the use of peening for ceramics is problematic as brittle materials may not exhibit significant plastic deformation and, accordingly, the development of residual stresses required for improved fracture toughness (Kic) may not be possible. However, conventional, mechanical shot peening has been shown to be feasible for some ceramics, such as SN-N320X and A61, using very specific processing conditions, although variability in the process may potentially introduce damage.

Little research has been carried out on the effect of LSP on these industrial materials, but it has been found that LSP can introduce cracks in the surface and cause catastrophic fracture of the brittle materials, which have a very limited range to achieve deformation and residual stress generation.

It is an object of this invention to alleviate at least some of the problems experienced with existing methods or improving the cutting edge fracture toughness of brittle materials, and in particular existing methods of performing laser shock peening on cemented carbides.

It is a further object of this invention to provide a method of inducing residual compressive stresses in brittle materials, in particular to provide a method of performing laser shock peening on brittle materials, which will be a useful alternative to existing methods. It is yet another object of this invention to provide a tool, such as a cutting tool, that will be a useful alternative to existing tools.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention there is provided a method of improving the fracture toughness or a brittle material, such as a cermet, including: selecting a cermet which has a fracture toughness between about 6 and about 15 MPa.m1 / 2 and Vickers hardness between about 9 and about 17 GPa; applying a coating to the cermet; and creating a hardened surface layer by means of laser shock peening, continuously improving the fracture toughness and hardness of the cermet.

The method may include inducing residual compressive stresses in a substrate of the cermet.

The energy being delivered to the substrate during the LSP process may be in the range of about 300 mJ to about 600 mJ. Preferably, the energy being delivered to the substrate during the LSP process is in the range of about 410 mJ to about 440 mJ.

The spot size of the laser used in the laser shock peening may be between about 0.7 to about 1.2 mm.

The LSP process may have a pulse duration of between about 0.5 ns and about 50 ns (FWHM), preferably between about 7 and about 10 ns, more preferably about 8.6 ns.

The LSP process may have a power intensity or between about 1 and about 20 GW / cm2, preferably between about 7.5 and about 8.5 GW / cm2.

The LSP process may have an overlap or between 0 and 90%.

The coating may be a sacrificial thermo-protective coating.

The method may include using an inertia containment medium in the form of a laser transparent medium, such as water, for example.

The method may include the step of producing the cermet by means of rapid sintering, preferably in the form of spark plasma sintering (SPS).

The cermet may be a cemented carbide.

The cermet may be selected from the group consisting of WC-X-12Co, NbC-X-12Co and NbC-X-12Ni, where X is either one or a combination of Cr3C2, Mo2C, TiC, SiC, TaC and VC.

In accordance with a second aspect of the invention there is provided a tool including a core made from a brittle material, such as a cermet, which has a fracture toughness between 6 and 15 MPa.m1 / 2, a surface layer hardened by laser shock peening, where the hardened surface layer has an increased fracture toughness compared to the fracture toughness of the core.

The tool is preferably a cutting tool.

The cermet may be a cemented carbide.

The cermet may be selected from the group consisting of WC-X-12Co, NbC-X-12Co and NbC-X-12Ni, where X is either one or a combination of Cr3C2, Mo2C, TiC, SiC, TaC and VC.

In accordance with a third aspect of the invention there is provided a system for performing laser shock peening on a brittle material, such as a cermet, the system including a laser source capable of delivering between about 300 mJ to about 600 mJ to the cermet at a pulse duration of between about 0.5ns and about 50ns (FWHM) and a power intensity of between about 1 and about 20 GW / cm2.

The spot size of the laser produced by the laser source may be between about 0.7 to about 1.2 mm.

The laser source preferably delivers between about 410 mJ to about 440 mJ to the cermet.

The power intensity is preferably between about 7.5 and about 8.5 GW / cm2.

LETTER DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows an optical image of an NbC-12Co (wt%) cutting tool insert showing catastrophic failure or the cutting edge on (a) the rake due to mechanical failure;

Figure 2 shows SEM-BSE images of WC-12Co (wt%), showing WC (light), and Co (dark), sintered by (a) LPS and (b) SPS;

Figure 3 shows SEM-BSE images or NbC-12Ni (wt%), showing NbC (light), Ni (medium) and pores (dark), sintered by: (a) LPS and (b) SPS;

Figure 4 illustrates the relationship between Vickers hardness (HV30) and fracture toughness (Kic);

Figure 5 shows SEM-SE images of WC-Cr3C2-12Co (W1-L) produced by LPS, showing: (a) long radial cracks before LSP, and (b) shorter radial cracks after LSP;

Figure 6 shows SEM-SE images of NbC based C1-L cermets produced by LPS, showing: (a) long radial cracks before LSP, and (b) much shorter radial cracks after LSP;

Figure 7 shows SEM-SE images of NbC-based F1-S cermets produced by SPS, showing: (a) long radial cracks before LSP, and (b) shorter radial cracks after LSP;

Figure 8 shows SEM-BSE images of NbC based C1-L cermets produced by LPS, showing crack propagation: (a) before LSP, and (b) after LSP;

Figure 9 illustrates the relationship between Vickers hardness (HV3o) and fracture toughness (Kic) for LPS cermets, before and after LSP;

Figure 10 illustrates the relationship between Vickers hardness (HV30) and fracture toughness (Kic) for spark plasma sintered cermets, before and after LSP; and

Figure 11 illustrates the comparison of fracture toughness (Kic) or all the cermets used during testing, before and after LSP.

DETAILED DESCRIPTION OF THE INVENTION

Before any other of the invention are explained in detail, it is understood that the invention is not limited in its application to the details of construction and the arrangement or components set forth in the following description or illustrated in the following drawings. The invention is capable of other devious and being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used is for the purpose of description and should not be considered as limiting. The use of "including," "including," or "having" and variations said is meant to encompass the items listed thereafter and equivalents thereafter as well as additional items. It is noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the," and any singular use of any word, include plural referents unless expressly and unequivocally limited to one sponsor . As used, the term "include" and its grammar variants are intended to be non-limiting, such that recitation or items in a list is not to the exclusion of other like items that can be dispensed or added to the listed items. A non-limiting example or a method of improving the abrasive resistance of brittle materials will now be described. In the context of this description the term “brittle materials” describe materials that have a combination of Shetty's fracture toughness of between 6 and 15 MPa.m1 / 2 and a Vickers hardness of between 9 and 17 GPa. The method preferably includes introducing compressive stresses in the brittle material, such as ceramics, cemented carbides or cermets, by performing laser shock peening (LSP). In the exemplified method discussed in detail below, various cermets are used. These cermets are typically selected from the group consisting of WC-X-12Co, NbC-X-12Co and NbC-X-12Ni, where X is either one or a combination of Cr3C2, Mo2C, TiC, SiC, TaC and VC. In the method described under the discussion of the experimental results the cermets include WC-0.8Cr3C2-12Co, NbC-12Co and NbC-12Ni. However, it should be understood that the method is not limited to these cermets and could be used on other brittle materials. It is further envisaged that the cemented carbides or cermets described in this specification could find particular application in the manufacturing of cutting tools, mining tools and tools used in other industrial applications where wear resistance and fracture toughness are important criteria. In this specification the word tool should be interpreted to include an insert for the tool where the insert defines the cutting edge.

As mentioned above, the NbC-Co cermets produced by conventional liquid phase sintering (LPS) have a lower hardness and fracture toughness than WC-Co cermets, because of NbC grain growth. In order to reduce the grain size of the NbC a rapid sintering technique, such as spark plasma sintering (SPS), is used. It has been found that due to the reduction in grain size resulting from the SPS there is an improvement in hardness and abrasion wear resistance. However, a negligible change in K, c is achieved by the SPS process. Prior to conducing any surface treatment, such as laser shock peening, one way of improving the Kic is by alteration of the composition of the cermet. For example, the composition can be altered by complete or partial substation or Co binder with Ni. Nickel has a higher plasticity than Co and always retains the more ductile fee structure (which has no phase transformation like Co), which increases the Kic, but reduces hardness.

In the SPS process, powder compositions are consolidated in a spark plasma sintering furnace. The powders are poured into cylindrical graphs with inner and outer diameters of about 20.9 mm and about 40 mm respectively, and about 48 mm height. The composite powder assemblies are heated in a vacuum (typically about 2 Pa) in two steps with different sintering profiles, depending on the powder compositions, to achieve good densification. For example, WC-10Co (wt%) powders are first heated to 1 000 ° C at a eating rate or 200 ° C and min. To 1220 ° C at a heating rate or 100 ° C / min, and the temperature is held at 1 220 ° C for 5 minutes during sintering. A cooling rate of 200 ° C / min is applied to all cermets. The applied pressure is adjusted from 16 MPa to 30 MPa at 1000 ° C, and from 30 MPa to 50 MPa at 1220 ° C within 30 seconds. The pressure is then constant at 50 MPa throughout the rapid sintering cycle. Horizontal and vertical graphite papers are used to separate the powders from the die and punch set-up. Hexagonal boron nitride is placed on the graphite paper to prevent carbon diffusion from the graphite paper to the powders during sintering.

Next, the cermet's resistance to crack-based phenomena such as fatigue and stress corrosion cracking is improved by means of a surface treatment. The preferred method of surface treatment is in the form of laser shock peening (LSP). After sintering, the cermet is polished using standard metallographic procedures and LSP is performed on the cermet's polished surface.

An optimized combination of LSP parameters or laser power intensity, spot size and spot coverage is applied to the cermet using a thin water layer as the inertial confinement medium. The inventors have identified the following optimized laser shock peening process conditions resulting in compressive stresses being introduced into the cermet: 1. Energy: between about 20mJ to about 100,000 mJ, preferably between about 300mJ and about 600mJ, most preferably between about 410mJ to about 440 mJ; 2. Spot size: between about 0.7 to about 1.2 mm; 3. Pulse duration: about 0.5ns to about 50ns (FWHM), preferably between about 7 and about 10 ns, more preferably about 8.6 ns; 4. Power Intensity: about 1 to about 20 GW / cm2, preferably about 7.5 to about 8.5 GW / cm2; and 5. Overlay: from 0% to 90% overlap.

During the LSP process, a sacrificial thermo-protective overlay or coating is applied to the cermet. It has been found that PVC tape, preferably black, is particularly effective as a coating. A laser transparent medium, such as water for example, could also be used as an inertia containment medium during the LSP process.

Experimental Results

Experimental testing was carried out on cermets produced by the above process in order to compare their mechanical properties to cermets produced by conventional methods. Cermets with the lowest Kic were used as control samples during optimization of the LSP parameters to ensure that the induced compressive residual stress would not result in crack formation. The optimized combination of parameters was then applied to all the cermets.

The conventional cermets used in the experimental testing were produced by liquid phase sintering. In particular, the liquid phase sintering was performed by heating the powders in a vacuum (0.04 MPa) at an initial heating rate of 2.4 ° C / min up to 1200 ° C. At 1200 ° C, cobalt loss protection (CLP) was carried out by adding argon gas at a pressure of 0.37 MPa, and a heating rate 3.5 ° C / min up to 1430 ° C. The temperature was constant for 75 minutes, and for the last 20 minutes, hot isostatic pressing (HIP) was done at 4.4 MPa to eliminate all surface porosity. The furnace was then water cooled at a rate of 3.5 ° C / min.

The different sintered samples used in the experimental tests are shown in Table 1 below.

Table 1. Samples and sintering processes

F1 and C1 are NbC based cermets with proprietary carbide additives to improve their abrasive resistance properties.

Archimedes ’principle was used to determine the density of the sintered cermets. Microstructures of the cemented carbides were examined in a field emission scanning electron microscope, with energy dispersive X-ray spectroscopy. Vickers hardnesses (HV30) were measured on polished specimens before and after LSP, using a load of 30 N, calculating an average of five indentations on different regions of each cermet. The

following criteria for the accurate derivation of fracture toughness (Kic) using Shetty's equation were satisfied: c / a> 1.3 and 0.25 <Ha <2.5, where c is the crack length from the center of indentation to the crack tip, a is half diagonal length of identification and I is the difference between c and a.

Shetty's facture toughness was calculated using Equations (1) and (2):

where H = Vickers hardness (GPa) P = Applied force (Newton) I = Average crack length (mm)

Figure 2 (a) and (b) show SEM micrographs or WC-10Co (wt%), sintered by conventional LPS and SPS respectively. The LPS cermet (Fig. 2 (a)) had larger WC grains with smaller and more homogeneously distributed Co pools than the SPS cermet (Fig. 2 (b)). The finer WC grains in the SPS cermet were from the shorter sintering dwell time and lower sintering temperatures, which prevented continuous Ostwald ripening. The more homogenous Co binder distribution in the LPS cermet was due to the formation of the Coliquid phase during sintering, which enhanced WC solubility, as well as the capillarity action of the liquid phase between the pores during the secondary rearrangement stage or sintering.

During SPS, there is considerable variation in temperature from the center to the surface of the conducting particles (Co and Ni) when the pulse electric current has passed. The temperature at the contacting surface reaches very high values, up to several thousand degrees, and momentarily results in melting, followed by rapid solidification of the metal binder. The rapid solidification prevents the homogenous distribution of the binder phase, explaining the poor binder distribution in the SPS cermet. Similarly, FIG. 3 shows that the LPS NbC-12Ni (wt%) (Fig. 3 (a)) cermet had larger NbC grains than for the similar

composition produced by SPS. This trend was observed in all the cermets produced by both LPS and SPS. A few pores were observed in the microstructures of the NbC based compositions and were attributed to the oxygen impurities in the starting NbC powders (1.6 wt%) which created spherical pores during sintering, explaining the slightly lower density of the NbC based cermets compared to the WC cermets (see Table 2 below).

From Table 2 below and Figure 4 it can be seen that all the SPS cermets had a higher hardness than the similar composition samples produced by LPS. This is due to the finer WC and NbC grains, from the short sintering times and lower sintering temperatures. Both LPS and SPS WC-12Co (wt%) cermets had a higher hardness than all NbC-based cermets, irrespective of the sintering technique, due to the higher hardness of WC (22.4 GPa) compared to NbC (19.6 GPa). Substitution of Co with Ni in the NbC cermets reduced the hardness, because of nickel's lower hardness and higher plasticity than cobalt. All the LPS cermets had generally higher Kic than their SPS counterparts (Table 2 and Figure 4), because of better binder distribution from the secondary rear arrangement or grains during sintering.

Table 2. Densification and mechanical properties of all the samples

* S = spark plasma sintered and L = liquid phase sintered

Both SPS and LPS NbC-12Co (wt%) cermets had much lower Kic than the WC-12Co (wt%) cermets, although they had the same Co binder contents. The reduction in Kic was due to the poorer wetting of Co on NbC than on WC, and the lower solubility in Co or NbC than WC, leading to the formation of brittle interconnected NbC networks and poorer distribution of Co. Substitution of Co with Ni in the NbC based cermets significantly improved the Kic, due to the higher plasticity or Ni and its stable fcc structure. The C1-L cermet had the poorest combination of hardness and Kic because of the large NbC grains from LPS.

Next, the effect of laser shock peening (LSP) on the cermets was explored. Figures 5 to 7 show the Vickers hardness (HV30) indentations and crack lengths in the W1-L, C1-L and F1-S cermets before and after LSP. The indentation crack lengths were used to calculate the Shetty's fracture toughness, where longer crack lengths mean lower fracture toughnesses. All the cermets, irrespective of the sintering technique, had shorter crack lengths after laser shock peening (LSP) than before. Generally, the liquid phase sintered cermets had greater reduction in crack lengths than the cermets produced by SPS (Figures 5 to 7). However, the liquid phase sintered cermets with originally low Kic values (~ 7 MPa.rn172), such as C1-L (Figure 4), had greater reduction in crack length than liquid phase sintered cermets with higher Kic values, such as W1- L (Figures 5 and 6).

No change in crack propagation mode was observed before and after LSP. Transgranular crack propagation, which is the most critical fracture mode in cermets, was the main mode of fracture in all the cermets. Reduction in crack lengths after LSP without change in the crack propagation mode (Figure 8) was attributed to the induced compressive residual stresses (CRS) which was limited transgranular crack propagation.

Based on the experimental results it has been found that LSP improved the Kic of all the cermets, and maintained the hardness as shown in Figures 9 and 10. From Figure 11 it can be seen that the liquid phase sintered cermets with pre-LSP low Kic showed much higher increases in Kic (doubled) than the LPS cermets with pre-LSP high Kic (~ 10%). The significant increase in Kic is attributable to two possible reasons. Firstly, since samples with the lowest Kic were used to deduce the optimization parameters, cermets with pre-LSP higher Kic might achieve higher Kic under more aggressive LSP conditions. Although the samples with the lowest pre-LSP Kic that had surface cracks under more aggressive LSP conditions, the samples with higher pre-LSP Kic might not have cracks under the same conditions due to higher toughness, leading to higher induced compressive residual stress and increased Kic. Secondly, it is possible that a maximum Kic (threshold) exists for a particular cermet composition, since very short cracks (no cracks in some cases) were observed at the identification edges or cermets with pre-LSP high Kic values (N2-L and F1-L) after LSP, indicating maximum Shetty's Kic. Generally, the liquid phase sintered cermets had a higher Kic than similar composition samples produced by spark plasma sintering (Fig. 11) because of the higher hardness of the spark plasma sintered cermets. For example, the N1-S cermet was ~ 3 GPa harder than N1-L (Table 2 and Fig. 4) after LSP, and had -20% increased Kic compared to N1-L which had doubled Kic (Fig. 11). The higher hardness in the SPS cermets may have limited the amount of plastic deformation during LSP, constantly reducing the toughening effect or the induced compressive residual stress.

In cermets, there is an inverse relationship between hardness and Kic, so increased hardness generally results in reduced Kic. However, by following the process according to the invention, in particular the combinations of rapid sintering (SPS), composition differences (substitution of WC by NbC and Co by Ni) and surface treatment (LSP), it is possible to increase both hardness and Kic in the same cermet.

From the above description of the experimental results it should be understood that spark plasma sintering (SPS) for higher hardness than conversional liquid phase sintering (LPS) for the same cermet composition. Both LPS and SPS WC-12Co (wt%) cermets had higher hardnesses than all the NbC based cermets, due to the higher hardness of WC than NbC, higher solubility or WC in Co than NbC in Co, as well as NbC in Ni. Substitution of Co by Ni in the NbC cermets increased the Kic due to the higher plasticity or Ni than Co and the former's stable fee structure. Advantageously, Laser shock peening (LSP) increased all Kic values, irrespective or composition and sintering method. In liquid phase sintered cermets with low Kc (about 6 to 7 MPa.m1 / 2) and low hardness (about 9 to 10 GPa), an increase of about 100% in Kic was observed. The laser shock peened SPS cermets had an increase of about 20 to 30% in Kic due to their high hardness limiting plastic deformation during LSP, while cermets with initial high Kic values (about 14 to 15 MPa.m1 / 2) had the least increase in Kic or about 10%.

From the above description of the method of increasing the fracture toughness of brittle materials, it should be clear that spark plasma sintering (SPS) results in the refinement of the microstructure, increasing the hardness compared to similar materials produced by conventional liquid phase sintering ( LPS). It has also been found that the substitution of WC with NbC reduces hardness, while the substitution of Co with Ni in the NbC-based cermets increases the Kic, but reduced the hardness. However, the inventors have identified a set of laser shock peening (LSP) parameters that allows LSP to be successfully performed on brittle materials, such as cemented carbides and cermets. From the abovementioned discussion of the experimental results it can be seen that LSP was successfully performed on all the cermets, which in an increased Kic while maintaining the hardness. After performing LSP on liquid phase sintered cermets, which initially had low Kic values in the region or 6 to 7 MPa.rn172, the Kic was doubled. The SPS cermets on the other hand has shown an increase in Kic or about 20 to 30% after LSP. The NbC-Ni cermets, which initially had high Kic values in the region of about 14 to 15 MPa.rn172, has shown the lowest increase in Kic or about 10%. Using rapid sintering, alteration or cermet composition and in particular LPS, it was shown that it is possible to improve both hardness and fracture toughness in cermets.

It is envisaged that the above-mentioned method in accordance with the invention could be used in manufacturing or treating or manufactured from brittle materials such as cemented carbides and cermets. For example, the tools could be in the form of cutting elements, cutting element inserts, drilling tools, drilling tool inserts, mining tools, wear parts in the mining and machining industries or any other tool where hardness and fracture toughness are important criteria. By carrying out the abovementioned method, in particular the step of performing LSP on a tool which has a core made from a cemented carbide or cermet material having a fracture toughness of between about 6 and 14 MPa.m1 / 2, a hardened surface layer, which has an increased fracture toughness compared to the fracture toughness or core, has been obtained. The increased fracture toughness of the hardened surface layer acts to increase the tool's resistance to fracturing.

It will be appreciated that the above is only one example of the invention and that there may be many variations without departing from the spirit and / or the scope of the invention. It is easily understood from the present application that has the particular features of the present invention, as generally described and / or illustrated in the figures, can be arranged and designed according to a wide variety of different configurations. In this way, the description of the present invention and the related figures are not provided to limit the scope of the invention but simply represent selected exponent.

The skilled person will understand that the technical characteristics of a given embodiment can be combined with characteristics of another embodiment, unless otherwise expressed or it is evident that these characteristics are incompatible. Also, the technical characteristics described in a given embodiment can be isolated from the other characteristics of this embodiment unless otherwise expressed.

Claims (28)

CONCLUSIONS A method for improving the fracture toughness of a brittle material comprising: selecting a brittle material having a fracture toughness between about 6 and about 15 MPa.m ^1/2 and a Vickers hardness between about 9 and about 17 GPa; applying a coating to the brittle material; and providing a cured surface layer by means of laser shock peening, thereby improving the fracture toughness and hardness of the brittle material. A method according to claim 1, comprising inducing residual compressive stresses in a substrate of the brittle material. The method of claim 1 or 2, wherein the energy delivered to the brittle material during the LSP process is in the range from about 300 mJ to about 600 mJ. The method of claim 1 or 2, wherein the energy delivered to the brittle material during the LSP process is in the range from about 410 mJ to about 440 mJ. The method of any one of claims 1 to 4, wherein the spot size of the laser used in laser shock beating is between about 0.7 to about 1.2 mm. The method of any one of claims 1 to 5, wherein the LSP process has a pulse duration ranging from about 7 ns to about 10 ns (FWHM). The method of any one of claims 1 to 5, wherein the LSP process has a pulse duration of about 8.6 ns (FWHM). The method of any one of claims 1 to 7, wherein the LSP process has a power density of between about 1 and about 20 GW / cm ² . The method of any one of claims 1 to 7, wherein the LSP process has a power density of between about 7.5 and about 8.5 GW / cm ² . The method of any one of claims 1 to 9, wherein the LSP process has an overlap between 0 and 90%. The method of any one of claims 1 to 10, wherein the coating is a sacrificial thermo-protective coating. The method of any one of claims 1 to 11, comprising using an inertial limiting medium in the form of a laser transparent medium. The method of claim 12, wherein the laser transparent medium is water. A method according to any one of claims 1 to 13, comprising producing the brittle material by rapid sintering. The method of claim 14, wherein the rapid sintering is in the form of spark plasma sintering (SPS). The method of any one of claims 1 to 15, wherein the brittle material is a cermet. The method of claim 16, wherein the cermet is a cemented carbide. The method of claim 16 or 17, wherein the cermet is selected from the group comprising WC-X-12Co, NbC-X-12Co and NbC-X-12Ni, wherein X is either one or a combination of CrsC2, M02C, TiC, SiC, TaC and VC. A tool comprising a core made of a brittle material having a fracture toughness between 6 and 15 MPa.m ^1/2 , a surface layer cured by laser shock peening, the cured surface layer being has increased fracture toughness compared to core fracture toughness. The tool of claim 19, wherein the tool is a cutting tool. The tool of claim 20, wherein the brittle material is a cermet. The tool of claim 21, wherein the cermet is a cemented carbide. The tool of claim 19 or 20, wherein the cermet is selected from the group comprising WC-X-12Co, NbC-X-12Co and NbC-X-12Ni, wherein X is either one or a combination of Cr ₃ C2, M02C, TiC, SiC, TaC and VC. 24. System for performing laser shock peening on a brittle material, the system comprising a laser source capable of delivering between 300 mJ to about 600 mJ to the brittle material at a pulse duration of between about 7 ns and about 10 ns (FWHM) and a power density between about 1 and about 20 GW / cm ² . The system of claim 24, wherein the spot size of the laser produced by the laser source is between about 0.7 to about 1.2 mm. 5 The system of claim 24 or 25, wherein the laser source is capable of delivering about 410 mJ to about 440 mJ on the cermet. The system of any one of claims 24 to 26, wherein the power density is between about 7.5 and about 8.5 GW / cm ² . The system of any one of claims 24 to 27, wherein the pulse duration is approximately 10 is 8.6 ns. 1 ⁷ 2 Fracture toughness, K _IC (MPa, m ') if) Ο) LL ⁶ / π Indentation co W cö P ^ O Pk u cö s ~ <o P O * f “4 -4 ~ b cö bfi cö P < O Lk (¾ M ' Q CÖ <30 .0 LL